The invention relates to devices to aid blind or vision-impaired persons to orient themselves in relation to surrounding objects.
The invention is based on the knowledge that most people posses an innate ability to orient themselves in relation to their surroundings by means of their binaural sense of hearing. This sense enables a person to perceive echoes coming in various directions from walls and obstacles. This ability, however, is only marginally useful to people mainly due to two problems, namely that the initial sounds that create the echoes momentarily "numb" the sensitivity of the ear so that the echo which is much more faint is not heard distinctly, and secondly that each echo, when reflected from nearby objects, arrives so short a time after the initial sound that the time difference is imperceptible. Several inventors have disclosed acoustic devices for aiding blind or vision-impaired persons by acoustic means. For example, U.S. Pat. No. 2,500,638 discloses a system of supersonic pulse transmission, wherein the reflected echoes are modulated to a pitch that depends on the distance to reflecting obstacles. U.S. Pat. No. 4,761,770 discloses a supersonic echo locating system wherein reflected pulses are described as being stretched in time by a certain factor, e.g. 32, thereby increasing the delay of received echoes to improve and enhance the perception of the echoes and thereby the distance to reflecting objects. In order to make the stretched echoes better perceptible, they are modulated by a white noise signal. The echo locating systems of the prior art, however suffer from the drawback that simply stretching the time axis for reflected echoes by a given factor does not simultaneously provide good perception of echoes from nearby objects and more distant ones because objects only few feet away must be stretched significantly in order to be perceived as being separate from the originating sound, while echoes from more distant objects become unreasonably stretched and thereby lose the character of an echo. In addition the stretched echoes must be modulated to be audible, which causes a blurring in the perception of overlapping echoes.
U.S. Pat. No. 4,907,136, of which the instant disclosure is a further development, discloses a system based on providing time-stretching means that include the sum of a fixed and a variable delay to the echo signal, wherein separate apparatus is provided for each of these functions. The instant application shows that these functions can be combined into common apparatus and that still better performance can be attained with common apparatus with a reduced investment in components, space and current drain. It is accordingly an object of the instant invention to overcome the drawbacks of the known art and thereby provide an acoustic echo-locating system for vision-impaired persons, which provides simultaneously improved distance perception of nearby as well as more distant objects and at the same time enables a blind person to form in his or her mind a mental "sound image" of the surroundings.